Servo unit



1943- B. G. CARLSON 2,454,551-

SERVO UNIT Filed March 2, 1945- INVENTOR. BERT G. Caruso/v ATTORNEY Patented Nov. 23, 1948 UNITED. "STATES PATENT OFFICE SERVO UNIT Bert G. Carlson, Gates Mills, Ohio, asslgnor to William R. Jack, Cleveland, Ohio Application March 2, 1945, Serial No. 580,608

. 4 Claims. 1

This invention relates to improvements in hydraulic servomotor units and has for one of its primary objects to provide a simple and emcient self-containedservomotor unit employing a servo cylinder and a piston hydraulically operated therein by fluid pressure leads on either side of the piston with a single valve means that is hydraulically operated to function as a combined by-pass, overpower and on-oif valve to replace the several valves usually present in servo units, particularly those used in connection with automatic pilots.

With the foregoing and other objects in view the invention resides in the combination of parts and in the details of construction hereinafter set forth in the following specification and appended claims, certain embodiments thereof being illustrated in the accompanying drawings, in which:

Figure l is a view in longitudinal section taken through the servomotorv unit showing the servo piston hard over to the right end of the cylinder and showing diagrammatically the hydraulic system connected to the servomotor unit; v

Figure 2 is a similar view showing the piston substantially midway between the cylinder ends and the overpower ball valve depressed and the by-pass between the two hydraulic power lines opened; and

Figure 3 is a view in section taken along line 3-3 of the servomotor unit of Figure 1.

Referring more particularly to the drawings, the casing I is provided with the usual smooth bore cylinder liner 2 in which the piston 3 is mounted to be reciprocated. -The piston rod 4 is connected by suitable means to the work desired to be done such as to the control cables for operating the control surfaces of an aircraft.

Heretofore, in automatic pilot systems such as shown in the United States patents to Kenyon, No. 2,210,916 and 2,210,917 of August 13, 1940, it has been conventional practice to have a fluid pressure lead from the balanced oil valve to either side of the servo piston and a valve controlled bypass in the servo unit between the two pressure leads. This conventional valve control involves a plurality of Spring operated overpower valves and a manually operated on-oil valve.

The two operating pressure lines I8 and I9 from the balanced oil valve 2I in casing 22 are connected .respectively to the unit to threaded 2 videdwith a bore to receive a bushing II to slidably receive a ball I2 in fairly snug relationship. Body portion III is screw threaded to receive a plug I3 between the bottom of which and the ball I2 is disposed a compression coil spring I4. The plug I3 is centrally bored which bore I5 is screw threaded to receive a lead 20 from a fluid pressure source of the hydraulic system.

Lead 20 connects with lead 23 through pressure regulator 24 to lead 25 intocasing 22. Lead 20 also communicates with lead 26 through constant pressure valve 21 and through pump 28 through lead 29 to sump 30. Another lead 3| extends from casing 22 to sump 30. The flow through lead 20 is controlled in an on or off, or by-pass, position by a three-way valve 32.

The strength of the spring I4 is relatively small and only relied upon to maintain little more if any than the actual weight of the bal1 I2. Under the control of any suitable three-way valve 32 which is turned on to supply pressure from the hydraulic system through opening I-5 the ball I2 remains seated in a fluid tight relationship, as shown in Figure 1, to close the by-pass 1 between openings 5 and 6 and 8 and 9 of cylinder chambers I6 and I1. The bore of the bushing II is relatively greater than that of the by-pass I so that while the unit hydraulic pressure is uniform through opening I5 as compared to that in opening 5 or 6 as transmitted to by-pass I, the over-all pressure on the greater exposed surface of ball I2 in bushing II is greater than that on the lesser exposed surface of the ball in by-pass 1 so as to maintain the ball I2 in its position of Figure 1 to close by-pass I.

As long as ball l2 keeps by-pass I closed the servo piston operates hydraulically in its cylinder. For instance, Figure 1 shows the piston forced out of normal neutral position to one hard over to the right. This condition has been brought about by the automatic hydraulic unequalization of pressure within cylinder chambers I5 and N. This, in turn has been caused by a pressure differential in pressure lines leading in at 5 and 6. Thus as a pressure differential has occurred in the automatic pilot balanced oil valve to open pressure flow in the lineleading in at 5 and to close the pressure flow to the line leading in at 6, the latter becomes the drain or return flow line, as indicated by the arrows. The movement of the piston to this position also moves the piston rod to do its work, such as operating the control surface of an aircraft.

If the pressure line leading in at I5 is valved to cut oil the supply of hydraulic pressure, the ball only sufficient to support the weight of the ball,

Any residual pressures tending to remain in line 20, are removed by the sump lead from valve 32 as passages 39 and 40 of said valve'are brought into alignment with leads 20 and 38 respectively by the counterclockwise closing movement of rotary element 43 in valve 32. Thus, the creation of any appreciable pressurediflerential in the two lines and 6 or the two cylinder chambers causes the ball I2 to become depressed against the slight action of spring I4 and open the by-pass I to render the servo ineffective as such. This constitutes ball I2 an on-oil valve, as far as the unit is concerned.

Should the aircraft operator desire to operate the manual control mechanism for operating the aircraft control surfaces through the servo unit without first neutralizing the same by cutting oil hydraulic pressure in the line leading in at I5 this may be done which results in the manual movement of piston rod 4 in the appropriate direction. Figure 2 shows the relative positions of the piston and ball, and by arrows the hydraulic flow which occurs as piston rod 4 is manually moved to the left from its position of Figure 1 to that of Figure 2, and further. In its neutral position the piston has equal pressure force at 5, 6 and I5 with the lands of the balanced oil valve closing the lines I8 and I 9 to the servo piston as shown in Figure 2. By a movement of the spool in the balanced oil valve as shown in Figure 1, the lands connect line I 8 to the pressure side and line I9 to the drain or return side of the pressure system while the hydraulic pressure in line 20 keeps the ball I2 seated in by-pass I. This causes a pressure differential in chamber l6 which forces piston 3 to the right, as shown in Figure 1.

Movement of the spool in the Opposite direction in the balanced oil valve would reverse the pressure side to line l9 and chamber I1 causing the piston to move to the left forcing the fluid out of chamber I6 into line I8 and thereby into the return side of the pressure system while ball I 2 still remains seated in bY-Dass I.

Should the operator want to overpower the system manually without turning the on-off valve off he could move the piston rod to the left and cause a difference of pressure in chamber I6 which would force the fluid into the by-pass chamber I (as lines I8 and I9 are of equal pressure) and depress the ball I2 into the bushing I I thereby overcoming the upward hydraulic pressure on ball I2 causing an opening in by-pass 1 to allow the fluid into chamber IT. The fluid in line 20 would be forced back into the pressure regulator and thereby into the sump.

Due to there being only an inch and threefourths stroke in the piston as chamber I6 is being evacuated of fluid and there not being the same displacement area in chamber I'l' (due to the piston rod) the ball I2 will be forced down further into bushing I I to take the excess fluid in that cycle of the stroke. As distinguished from the usual two overpower valves conventionally used whichrely entirely on spring pressure to keep them normally closed and which springs are susceptible to unequal strength and variations due to thermal changes, the ball I2 is a single valve dependent only upon the hydraulic pressure in the hydraulic system to function reliably and efllciently as a virtually springless combined bypass, on-ofi and overpower servomotor unit valve.

from said hydraulic pressure source, a ball valve slidably mounted in said lead and extending into said by-pass in a fluid tight relationship with said by-pass, said intermediate lead being of greater cross-sectional area than that of said by-pass to permit the exertion of a relatively greater overall hydraulic pressure to maintain said ball valve closed, said ball valve being adapted to be opened upon the existence of an abnormal pressure in said by-pass in excess of that in said intermediate lead.

2. In a servomotor unit, a power cylinder and a piston reciprocally mounted therein, a source of hydraulic pressure with leads to said unit and through cylinder ports to cylinder chambers on either side of said piston, a by-pass between said pressure leads, an intermediate lead in said unit from said hydraulic pressure source, a ball valve slidably mounted in said lead and extending into said by-pass in a fluid tight relationship with said by-pass, said intermediate lead being of greater cross-sectional area than that of said by-pass to permit the exertion of a relatively greater over-all hydraulic pressure to maintain said ball valve closed, said ball valve being adapted to be opened upon the existence of an abnormal pressure in said by-pass in excess of that in said intermediate lead, and valve means for cutting oil. and on the supply of hydraulic pressure to said ball valve.

3. In a servomotor unit, a power cylinder and a piston reciprocally mounted therein, a source of hydraulic pressure with leads to said unit and through cylinder ports to cylinder chambers on either side of said piston, a by-pass between said pressure leads, an intermediate lead in said unit from said hydraulic pressure source, a ball valve slidably mounted in said lead and extending into said by-pass in a fluid tight relationship with said by-pass, said intermediate lead being of greater cross-sectional area than that of said by-pass to permit the exertion of a relatively greater overall hydraulic pressure to maintain said ball valve closed, said ball valve being adapted to be opened upon the existence of an abnormal pressure in said by-pass in excess of that in said intermediate lead, created by the manual operation of said piston in opposition to the existing normal hydraulic pressures in said cylinder chambers.

4. In a servomotor unit, a power cylinder and a piston reciprocally mounted therein, a source of hydraulic pressure with leads to said unit and through cylinder ports to cylinder chambers on either side of said piston, a by-pas's between said pressure leads, an intermediate lead in said unit from said hydraulic pressure source, a ball valve slidably mounted in said lead and extending into said by-pass in a fluid tight relationship with said by-pass,'said intermediatelead being of greater cross-sectional area than that of said by-pass to permit the exertion of a relatively greater overall hydraulic pressure to maintain said ball valve closed, said ball Valve being adapted to be opened hydraulic pressure to said ball valve and for cutting of said supply to permit hydraulic operation of said ball valve into a. position to open said bypass to render said servo ineflective upon the presence of any appreciable shift of hydraul pressure diflerential in said cylinder. 7

The following references are of record in-the file of this patent:

Number UNITED STATES PATENTS Name Date Johnson Oct. 23, 1906 Carlson Rh. 2, 1937 Fischel Nov. '1, 1939 Dodson May 19, 1942 

